1. Field of the Invention
The present invention relates to an information reproducing apparatus and an information recording medium, and more particularly relates to an information recording medium in which a shape of an information pit is changed by multiple steps in accordance with a value of digital information to be recorded and accordingly the digital information is multi-level-recorded, and an information reproducing apparatus for reproducing the digital information, which is multi-level-recorded, from the information recording medium.
2. Description of the Related Art
There is a research with regard to a technique for recording or reproducing multi-value digital information having a plurality of values in addition to xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d onto or from an information recording medium in order to record or reproduce a large amount of digital information onto or from an optical disc as an information recording medium at a high density.
As a conventional technique in this field, an information recording and reproducing method employing a so-called xe2x80x9cpit edge multi-level recording and reproducing methodxe2x80x9d is known. This multi-level recording and reproducing method is to record the digital information by changing a position of an edge (implying both ends of an information pit of an optical disc existing in a rotation direction thereof, and hereafter implying the same) of the information pit formed at a certain cycle on a track on an information record surface of the optical disc, by multiple steps in the rotation direction in accordance with a value of the digital information to be recorded, and to reproduce the digital information recorded in this manner from the information record surface of the optical disc. For example, the information recording and reproducing method employing this pit edge multi-level recording and reproducing method is disclosed in detail in Japanese Patent Application Laid Open No. Hei.6-76303.
Here, at a time of reproduction in the pit edge multi-level recording and reproducing method, a reproduction light beam is irradiated with respect to the pit edge of the optical disc, on which the digital information is multi-level-recorded by changing the position of the pit edge by multiple steps. Then, a multiple level judgment using a plurality of preset thresholds is performed with respect to an analog detection signal obtained by receiving the reflection light from the pit. Accordingly, the digital information is reproduced.
Next, the operation at a time of reproduction in the pit edge multi-level recording and reproducing method will be described below with reference to FIG. 14. FIG. 14 is a diagram describing the operation in which the digital information is reproduced by irradiating a reproduction light beam to only one track at a time.
As shown on a top stage of FIG. 14, in the conventional pit edge multi-level recording method, an information pit row is constituted by information pits PT on a track TR at a constant cycle, and a position of a pit edge at each information pit PT is changed, for example, by three steps (in case that the digital information has a multi-level of three values) in a rotation direction of the optical disc, in accordance with the above-mentioned value of the digital information. In addition, on the top stage of FIG. 14, the positions that can be considered as the position of the pit edge in each information pit PT are all shown by using solid lines. However, a pit edge is actually formed at any one position among those three pit edge positions (similarly in FIG. 15 described later).
On the other hand, when the reproduction light beam is irradiated with respect to only one track at a time to thereby reproduce the digital information, the light beam irradiated to the optical disc generates a beam spot BS having an irradiation range indicated by a circle on the top stage of FIG. 14. Then, since this beam spot BS is moved on the track TR indicated by a solid line, it is possible to sequentially read the digital information that is multi-level-recorded in one pit edge.
At this time, one track signal indicative of a position of the information pit on the optical disc (in other words, a positional relation between a land area in which the information pit is not formed and an area in which the information pit is formed) is alternately changed between a positive area (indicative of a range of the information pit) and a negative area (indicative of a range of the land area) in accordance with a position of each pit edge, as shown on a second stage from the top stage of FIG. 14. Also, a beam profile indicative of a change of an irradiation strength of the light beam with the beam spot BS on the optical disc as a center has the distribution as shown on a second stage from a bottom stage of FIG. 14.
When the light beam is irradiated in the manner as shown on the top stage to the second stage from the bottom stage of FIG. 14, an analog detection signal detected on the basis of the reflection light has a waveform as shown on the bottom stage of FIG. 14. At this time, a waveform of the analog detection signal corresponds to a waveform, in which the waveform indicative of the beam profile and the one track signal are multiplied by each other.
Incidentally, the bottom stage of FIG. 14 shows a plurality of kinds of waveforms having the possibility that they may be detected as the analog detection signal in such a way that all of them overlap with each other. However, the actual analog detection signal has one waveform among those waveforms shown on the bottom stage of FIG. 14. Moreover, the bottom stage of FIG. 14 shows the case that a level of the analog detection signal is made higher when the information pit PT is read in (similarly in FIG. 15 as described later).
At this time, as shown on the bottom stage of FIG. 14, an output analog detection signal is separated into multiple levels corresponding to the positions of the pit edge, at a timing corresponding to an average change point from the information pit to the land area or from the land area to the information pit (this phenomenon is typically referred to as an xe2x80x9cEye Openxe2x80x9d). Then, the analog detection signal ise sampled at the timing corresponding to this average change point. Moreover, the sampled values are threshold-judged on the basis of the two thresholds shown on the bottom stage of FIG. 14. Accordingly, the value of the digital information recorded in each pit edge is recovered/reproduced.
According to such a pit edge multi-level recording and reproducing method, the digital information can be recorded or reproduced at a density higher than that of the conventional case of recording or reproducing the digital information in which only xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d are used as their values.
However, in the above-mentioned conventional pit edge multi-level recording and reproducing method of example, if an angle between an optical axis of the reproduction light beam and an information record surface on which the information pit PT is formed is deviated from a right angle because of deflection or distortion of the optical disc (hereafter, this drift is referred to as a xe2x80x9ctilt driftxe2x80x9d), the shape of the beam spot BS formed on the information record surface of the optical disc is deformed to thereby mix a level fluctuation component into the analog detection signal at the time of the reproduction. Moreover, if the analog detection signal is threshold-judged as mentioned above in the condition that the level fluctuation component is mixed, this leads to a problem that an error is induced in the judged result.
That is, as shown in FIG. 15, when the tilt drift causes the shape of the beam spot BS formed, on the information record surface to be deformed (e.g., elliptically deformed in FIG. 15), the symmetry with a line passing through the central point of the beam spot BS in the beam profile as a symmetric axis is destroyed (refer to the second stage from the bottom stage of FIG. 14 and a second stage from a bottom stage of FIG. 15). Thus, the waveform of the analog detection signal resulting from the product of the waveform indicative of the beam profile and the one track signal contains the level fluctuation component as shown on the bottom stage of FIG. 15.
Actually, the deformation of the symmetry in the beam profile causes the deformation of a linear phase property in a phase transmission performance of a reproduction system in the pit edge multi-level recording and reproducing method. So, the phase transmission performance contains the phase fluctuation. As a result, a position of an average zero-crossing point of the actual analog detection signal is deviated from an original position so that the level fluctuation component is included in even the sample value corresponding to its sampling timing.
Then, the change of the values when the analog detection signals in this condition are sampled at the sampling timing equal to that of the case shown in FIG. 14 is undulated uprightly and downwardly as compared with the case of FIG. 14, as shown on the bottom stage of FIG. 15.
Thus, if the sampled values containing those undulations are threshold-judged in accordance with the threshold similar to that of the case of FIG. 14, a judgment value is outputted which is different from a value of the digital information actually recorded in a corresponding pit edge. Accordingly, this results in an occurrence of a so-called xe2x80x9cdecode errorxe2x80x9d.
By the way, it is confirmed by the inventor of the present invention that the undulation caused by the level fluctuation component contained in the analog detection signal has a larger influence on the increase of the recovery error as compared with a so-called xe2x80x9cintra-code interferencexe2x80x9d that is typically included at the time of the reproduction of the digital information.
The present invention is proposed in view of the above mentioned problems. It is therefore an object of the present invention to provide an information reproducing apparatus and an information recording medium, which can cancel out an influence of a level fluctuation component included in a generated analog signal to thereby accurately reproduce digital information, when the digital information is reproduced from an information recording medium on which the digital information is multi-level-recorded even if such a level fluctuation component is contained in the generated analog detection signal.
The above object of the present invention can be achieved by an information reproducing apparatus for reproducing digital information from an information recording medium such as an optical disc etc., on which the digital information is multi-level-recorded by changing a shape of an information pit in, correspondence with the digital information by multiple steps. The information reproducing apparatus is provided with: a detection signal generating device such as a light receiving unit etc., for generating an analog detection signal corresponding to the information pit; a sampling device such as an A/D converter etc., for sampling the generate analog detection signal at a sampling cycle corresponding to a cycle, at which the information pit is formed on the information recording medium, to thereby generate a sampling detection signal; an extracting device such as a shift register etc., for extracting a level fluctuating component, which is included in the analog detection signal, out of the generated sampling detection signal; a first subtraction signal generating device such as a subtracter etc., for subtracting the extracted level fluctuating component from the generated sampling detection signal, to thereby generate a first subtraction signal; and a reproducing device such as a decoder etc., for reproducing the digital information on the basis of the generated first subtraction signal.
According to the information reproducing apparatus of the present invention, when reproducing the multi-level-recorded digital information, since the level fluctuating component included in the corresponding analog signal is extracted and cancelled out, it is possible to accurately reproduce the multi-level-recorded digital information.
Therefore, even if the positional drift or the like at the time of forming the information pit occurs, which may cause the level fluctuating component, it is possible to reproduce the digital information accurately while canceling the evil effect of such a positional drift or the like.
In one aspect of the information reproducing apparatus of the present invention, the digital information is multi-level-recorded on the information recording medium by changing respective positions of two edges opposite to each other of each information pit, by multiple steps in correspondence with the digital information. The detection signal generating device is provided with: an irradiating device such as a reproducing head etc., for irradiating a reproducing light beam to reproduce the digital information onto one track, which is formed of a plurality of information pits on the information recording medium; and a light receiving device for receiving a reflection light of the reproducing light beam from each of the information pits, to thereby generate the analog detection signal. The level fluctuating component is included in the analog detection signal caused by a drift from a right angle of an angle between (i) an information recording surface, on which the information pit is formed, of the information recording medium and (ii) an optical axis of the reproducing light beam. The reproducing device reproduces the digital information by comparing a level of the first subtraction signal with a plurality of thresholds set in advance in accordance with multiple levels in multi-level recording of the digital information.
According to the information reproducing apparatus of the present invention, even if the level fluctuating component is included in the corresponding analog signal due to a drift of the angle between (i) an information recording surface of the information record medium and (ii) the optical axis of the reproducing light beam, from a right angle, it is possible to optically reproduce the multi-level-recorded digital information accurately.
In this aspect of the information reproducing apparatus of the present invention, the extracting device may be provided with: a shift register device such as a shift register etc., for sequentially delaying the generated sampling detection signal for each sampling cycle, to thereby output a delay signal respectively for each delay of one sampling cycle; an inverting device such as a shift register etc., for inverting even-numbered delay signals among delay signals outputted from the shift register device, to thereby generate inverted delay signals; an adding device such as an adder etc., for adding (i) odd-numbered delay signals among the delay signals outputted from the shift register device and (ii) the inverted delay signals respectively to each other, to thereby generate an addition signal; and a level signal generating device such as a divider etc., for dividing a level of the addition signal by the number of delay stages in the shift register device, to thereby generate a level signal indicative of a level of the level fluctuating component. The first subtraction signal generating device subtracts the generated level signal from the generated sampling detection signal, to thereby generate the first subtraction signal.
Accordingly, since the amplitude variation of the analog detection signal, which is caused by a mixture of the level fluctuating component into the analog detection signal, is averaged and reduced by the feed forward control, it is possible to accurately reproduce the multi-level-recorded digital information while canceling the level fluctuating component.
In this case, the number of delay stages in the shift register device may be an even number.
Accordingly, it is possible to prevent the detection accuracy from being deteriorated due to a direct current component generated in the process of averaging the amplitude variation of the analog detection signal.
Alternatively, in this one aspect of the information reproducing apparatus of the present invention, the extracting device may be provided with: an adding device such as an adder etc., for adding (i) the generated sampling detection signal and (ii) a multiplied delay signal, to thereby generate an addition signal; a delaying device such as a delay etc., for delaying the generated addition signal by one sampling cycle, to thereby generate a delay signal; a multiplying device such as a multiplier etc., for multiplying the generated delay signal by a multiplication coefficient, which is set in advance to have a value greater than xe2x88x921, to thereby generate and output the multiplied delay signal to the adding device; and a level signal generating device such as a multiplier etc., for multiplying the generated addition signal by an addition coefficient, which has a value that one is added to the multiplication coefficient, to thereby generate a level signal indicative of a level of the level fluctuating component. The first subtraction signal generating device subtracts the generated level signal from the generated sampling detection signal, to thereby generate the first subtraction signal.
Accordingly, since the amplitude variation of the analog detection signal generated by the mixture of the level fluctuating component into the analog detection signal can be averaged and reduced by the feed forward control without employing the shift register device or the like, it is possible to simply and accurately reproduce the digital information while eliminating the level fluctuating component.
Alternatively, in this one aspect of the information reproducing apparatus of the present invention, the extracting device may be provided with: a second subtraction signal generating device such as a subtracter etc., for subtracting a delay signal form the generated first subtraction signal, to thereby generate a second subtraction signal; a delaying device such as a delay etc., for delaying the generated second subtraction signal by one sampling cycle, to thereby generate and output the delay signal to the second subtraction signal generating device; and a level signal generating device such as a multiplier etc., for multiplying the generated second subtraction signal by a multiplication coefficient, which has a value less than 1, to thereby generate a level signal indicative of a level of the level fluctuating component. The first subtraction signal generating device subtracts the generated level signal from the generated sampling detection signal, to thereby generate and output the first subtraction signal to the reproducing device and the second subtraction signal generating device.
Accordingly, since the amplitude variation of the analog detection signal generated by the mixture of the level fluctuating component into the analog detection signal can be averaged and reduced by the feed back control without employing the shift register device or the like, it is possible to simply and accurately reproduce the digital information while eliminating the level fluctuating component.
In another aspect of the information reproducing apparatus of the present invention, the digital information is multi-level-recorded on the information recording medium by changing respective positions of two edges opposite to each other of each information pit, by multiple steps in correspondence with the digital information. A plurality of information pits are formed in a zigzag shape between two tracks adjacent to each other, as for a plurality of tracks formed of the information pits on the information recording medium. The detection signal generating device is provided with: an irradiating device for irradiating a reproducing light beam to reproduce the digital information onto the information recording medium such that an irradiation range of the reproducing light beam includes a plurality of edges on the tracks adjacent to each other; and a light receiving device for receiving a reflection light of the reproducing light beam from each of the edges on the tracks, to thereby generate the analog detection signal. The level fluctuating component is included in the analog detection signal caused by a drift of a position, where each information pit is formed, from a position corresponding to the cycle by which the information pit is to be formed. The reproducing device reproduces the digital information by comparing a level of the first subtraction signal with a plurality of thresholds set in advance in accordance with multiple levels in multi-level recording of the digital information.
According to this aspect, even if the level fluctuating component is included in the analog detection signal due to the positional drift or the like, it is possible to optically reproduce the digital information accurately.
In this another aspect of the information reproducing apparatus of the present invention, the information recording medium may comprise the information pits having standard edges, which are the edges each formed at a position corresponding to a standard level set in advance for the multiple levels and which are included simultaneously in the irradiation range. The extracting device may be provided with: a second subtraction signal generating device such as a subtracter etc., for subtracting a delay signal form the generated first subtraction signal, which is generated at a timing when only the standard edges are included in the irradiation range, to thereby generate a second subtraction signal; a delaying device such as a delay etc., for delaying the generated second subtraction signal by one sampling cycle, to thereby generate and output the delay signal to the second subtraction signal generating device; and a level signal generating device such as a multiplier etc., for multiplying the generated delay signal by a multiplication coefficient, which has a value less than 1, to thereby generate a level signal indicative of a level of the level fluctuating component. The first subtraction signal generating device may subtract the generated level signal from the generated sampling detection signal, to thereby generate and output the first subtraction signal to the reproducing device and the second subtraction signal generating device.
Accordingly, since the amplitude variation of the analog detection signal caused by the mixture of the level fluctuating component into the analog detection signal is averaged and reduced by the feed back control, on the basis of the subtraction signal corresponding to the standard pit, even if the digital information recorded on the edges striding over a plurality of tracks is reproduced simultaneously, it is possible to accurately reproduce the digital information while eliminating the level fluctuating component.
Alternatively, in this another aspect of the information reproducing apparatus of the present invention, the information recording medium may comprise the information pits having standard edges, which are the edges each formed at a position corresponding to a standard level set in advance for the multiple levels and which are included simultaneously in the irradiation range. The standard edge may be formed to be included in the irradiation range at two irradiation timings respectively corresponding to a start timing and an end timing of one sampling cycle. The extracting device may be provided with: a first delaying device such as a delay etc., for delaying the first subtraction signal, which is generated at a former irradiation timing of the two irradiation timings, by one sampling cycle, to thereby generate a first delay signal; a second subtraction signal generating device such as a subtracter etc., for subtracting the first delay signal from the first subtraction signal, which is generated at a latter irradiation timing of the two irradiation timings, to thereby generate a second subtraction signal; a third subtraction signal generating device such as a subtracter etc., for subtracting a second delay signal form the second subtraction signal corresponding to the latter irradiation timing, to thereby generate a third subtraction signal; a second delaying device such as a delay etc., for delaying the generated third subtraction signal by one sampling cycle, to thereby generate the second delay signal; and a level signal generating device such as a multiplier etc., for multiplying the generated second delay signal by a multiplication coefficient, which is set in advance to have a value greater than xe2x88x921, to thereby generate a level signal indicative of a level of the level fluctuating component. The first subtraction signal generating device may subtract the generated level signal from the generated sampling detection signal, to thereby generate and output the first subtraction signal to the reproducing device, the first delaying device and the second subtraction signal generating device.
Accordingly, since the amplitude variation of the analog detection signal caused by the mixture of the level fluctuating component into the analog detection signal is averaged and reduced by the feed back control, on the basis of the subtraction signal generated at the sampling timing corresponding to the standard pit, even if the digital information recorded on the edges striding over a plurality of tracks is reproduced simultaneously, it is possible to accurately reproduce the digital information while eliminating the level fluctuating component.
The above object of the present invention can be also achieved by an information recording medium, on which digital information having multiple levels is multi-level-recorded by changing a shape of information pit by multiple steps in accordance with the digital information, wherein the information pit is shaped in a zigzag shape between two tracks adjacent to each other, each track being formed of a plurality of information pits on the information recording medium, positions of two edges opposed to each other of each information pit are respectively changed by multiple steps in correspondence with the multiple levels, the information pits have standard edges respectively, which are formed at positions corresponding to a standard level set in advance in the multiple levels and are included simultaneously in an irradiation range of a light beam to reproduce the digital information on the information recording medium, and the digital information is reproduced from the information recording medium by detecting the standard edges by using the light beam while a level fluctuating component included in an analog detection signal corresponding to the information pit is cancelled out.
According to the information recording medium of the present invention, since the level fluctuating component is eliminated by detecting the standard edge, even if the level fluctuating component is included in the analog detection signal, it is possible to optically reproduce the multi-level-recorded digital information accurately.